This might sound strange but I've been using Quicken (came pre-installed on my laptop and I'm thinking it's pre-installed on a good many computers, because the rest of our computers have it, also). I have it set up as a "Cash" account, in the Ref. field I enter whether it was an instant win, 1x entry, etc. In the Payee field I enter the company sponsoring the sweeps; in the category field I enter "other income," and in the memo field I enter the prize. The approx. retail value is then entered in the Receive field. For the Date field, I enter the date I won, or was notified. Once a prize has been received, I mark it as "cleared." Categories can be added, so I may at some point edit them to reflect the type of sweepstakes entry...instant win, 1x, etc.
It works really well, you can print reports, graphs, compare winnings from year to year, month to month etc. etc. and it keeps a running balance with each new "transaction." There's also an address book, great for keeping sponsor's contact information (for those big wins), and you can set reminders or notes. You can do the same if you have any verson of Microsoft Money installed (that comes pre-installed on some computers also!)
I don't know that I would run out and buy it just for this (you can get the current version for about 29.95) but it is worth checking to see if it was pre-installed on your computer!